Plastic materials used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together and form a smooth coating.
Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above-stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough finish referred to as an "orange peel" surface. Such a coating surface or finish lacks the gloss and luster of coatings typically obtained from thermoplastic compositions. The "orange peel" surface problem has caused thermosetting coatings to be applied from organic solvent systems which are inherently undesirable because of the environmental and safety problems that may be occasioned by the evaporation of the solvent system. Solvent-based coating compositions also suffer from the disadvantage of relatively poor percent utilization, i.e., in some modes of application, only 60 percent or less of the solvent-based coating composition being applied contacts the article or substrate being coated. Thus, a substantial portion of solvent-based coatings can be wasted since that portion which does not contact the article or substrate being coated obviously cannot be reclaimed.
In addition to exhibiting good gloss, impact strength and resistance to solvents and chemicals, coatings derived from thermosetting coating compositions must possess good to excellent flexibility. For example, good flexibility is essential for powder coating compositions used to coat sheet (coil) steel which is destined to be formed or shaped into articles used in the manufacture of various household appliances and automobiles wherein the sheet metal is flexed or bent at various angles.
The concept of plasticizers in the plastics industry is well-known. Plasticizers are generally used to improve the melt flow during the extrusion and to modify the properties of the resulting plastic. In powder coating applications, plasticizers have been found to improve melt flow and thus improve appearance of the coating but they also lower the powder stability which is essential for a powder coating composition. Plasticizers are also known to migrate (i.e., "bloom") to the surface of the coating thereby causing a milky temporary thin film. Thus, caking resulting from storage instability and the blooming phenomenon limits the usefulness of known plasticizers in powder coating compositions.
Thus, the main objective of the invention is to provide such a plasticized thermosetting powder coating for improved overall appearance, better smoothness and gloss, less orange peel and better coating flexibility. A further objective of the invention is to accomplish improved properties without reducing powder stability. A further objective is the improvement in cure response of coating compositions through the use of a reactive plasticizer, particularly in the case of coating compositions based on less reactive crosslinking agents.
U.S. Pat. No. 4,988,767 describes a powder coating composition based on Beta-hydroxylalkyl amide curing agent and a first acid group containing acrylic polymer having a Tg of -20.degree. to 30.degree. C. and a second acid-group containing acrylic polymer having a Tg of 40.degree. to 100.degree. C. to form a crosslinked product.
U.S. Pat. No. 4,937,288 describes a thermo-setting powder coating composition which comprises a carboxylic group containing acrylic polymer having a Tg of 35.degree.-100.degree. C. and carboxylic acid group containing polyester which is crystalline under 50.degree. C. together with a Beta-hydroxylalkylamide curing agent. Preferred acids are dodecanoic acid, polymeric polyanhydride or low molecular weight polyesters with acid number of 75-375 and preferred melting point of 70.degree. to 110.degree. C.
U.S. Pat. No. 4,859,760 describes a powder coating composition containing an amorphous/semi-crystalline hydroxyl polyesters, compounded with diisocyanate crosslinkers to produce powder coatings with good balance of flexibility and hardness.
U.S. Pat. Nos. 4,442,270 and 4,387,214 describe a semi-crystalline copolyesters derived from terephthalic acid and 1,6-hexanediol or its variation for use in primers or overcoatings for automobiles.